Optical fiber systems are well known and offer several advantages, including a high bandwidth for signal transmission, low noise operation, and inherent immunity to electromagnetic interference. Because of these advantages, optical fibers are increasingly used in a variety of communications applications, including voice, video, and data transmissions.
Outdoor-use optical fiber cable frequently includes optical fibers enclosed within a buffer tube and surrounded by an external jacket to protect the optical fibers from environmental conditions. In addition, an optical fiber cable intended for outdoor use can further include longitudinally positioned reinforcing members, typically in the form of longitudinally extending rods, to provide tensile strength and structural rigidity along the length of the cable. The reinforcing members can limit axial tensile strain of the cable resulting from tensile forces, such as can occur during cable installation. The reinforcing members can also limit the axial thermal strain of the cable when the cable is subject to significant changes in temperature.
Optical fibers used for voice, data, and video transmission typically include a glass core, where the majority of the light signal travels, and a surrounding glass cladding, which serves as a waveguide to keep the light traveling axially in the core (i.e., because of the refractive-index differences between the glass cladding and the glass core). The glass core and cladding are surrounded by one or more polymeric coatings, which offer mechanical protection to the underlying glass cladding and glass core. Two or three protective coating layers are most common. In this regard, the innermost coating used is typically softer, relatively low modulus polymeric material to buffer the glass cladding and core from mechanical stresses. The outer coating is typically a higher modulus secondary coating that provides mechanical protection while facilitating handling of the optical fiber over the cabling, installation, and operating life of the optical fiber.
To effectively use optical fibers in a transmission system, there is a need to make connections of optical fibers at various points in the network. Examples of connection points that are commonly needed are (i) connections of individual optical fiber cable lengths to create a longer continuous optical fiber, (ii) connections to create branching points that reroute fibers in the same cable in different directions as needed to provide fibers at desired locations, and (iii) connections of active and passive components, such as (a) LEDs and lasers, which create the optical signals, (b) amplifiers and attenuators to increase or reduce the signal power, respectively, (c) detectors to detect and interpret the optical signals, and (d) optical splitters to multiply the number of optical signals.
Optical fiber connections are normally made by (i) fusion splicing where two ends of the optical fibers are welded together at glass contact points (and a protective sleeve placed over the weld point); (ii) mechanical splices where the two ends of fibers being joined are coupled together with a mechanical apparatus; or (iii) mechanical connectors where the two ends of fibers are coupled together with a mechanical apparatus. Those having ordinary skill in the art will appreciate that a mechanical connector is different from a mechanical splice in that the joining apparatus is designed to be connected, disconnected, and reconnected multiple times over the useful life of a connector while providing a high-quality, low-added-loss, low-optical-reflection joint between the connected optical fibers. In contrast, a mechanical splice is typically designed to be connected once and then stored over its operating life.
Historically, fusion-splicing joining technology has been preferred for connections made outdoors (i.e., in the outside-plant environment) because of the low power loss at the joint, the low signal reflection at the joint, and the permanency and long-term reliability of this welding technology. Fibers that are joined by fusion splicing machines must have a minimal fiber length of a few feet extending from the cable structure to facilitate placement into the fusion splicing machine for the welding process.
In order to keep fibers grouped in units for easier storage and handling, small, flexible, non-UV-resistant furcation tubes are placed over most of the length of extended fiber. These conventional furcation tubes are typically solid or spiral low-modulus polymeric tubes that provide a flexible means for grouping and handling fibers while providing little protection. Alternatively, in the case of some stranded loose-tube optical-fiber-cable designs, the buffer tube containing the fiber can be extended from the cable along with the fiber to provide the same fiber grouping and handling benefits with very limited protection (i.e., because of the low modulus of the tubes). These furcation or buffer tubes are then stored along with the fusion splice in a protective plastic or metallic enclosure to provide adequate protection from outdoor elements (e.g., extreme temperatures, wind, rain, snow, ice, and solar UV radiation).
Active and passive components have been conventionally located inside of buildings, remote terminals, or protective cabinets for protection from outdoor elements that can occur in the outside-plant environments. It has been common practice to utilize mechanical connectors in these more protective environments to make the necessary connections of optical fibers to other optical fibers or to active and passive components. Optical connectors are especially attractive when there is a likelihood of disconnecting and reconnecting optical fibers. Similar to the fusion splice situation, a length of fiber typically extends from the cable structure to allow both the installation of the connector and the routing of the connector to the appropriate position for the connection to be made. Also similar to fusion splices, this extended length of fiber is typically provided some limited protection by a furcation tube, which is subsequently stored in a protective cabinet or enclosure.
The mating ends of connectors may be installed onto the fiber ends either in the field (e.g., at the network location) or in a factory prior to installation into the network. The advantage of installing the mating ends of the connectors in a factory is that the connector installation process can be made faster, less expensively, and with a higher quality in a manufacturing environment than in a field environment. In the field, the ends of the connectors are mated in order to connect the fibers together or connect the fibers to passive or active components.
The advent of a market for high-bandwidth communication services/content to the home (e.g., high speed Internet access, cable television, high-definition television (HDTV), and video-on-demand) has created the need to reduce the costs and complexity of installing FTTH (Fiber-to-the-Home) networks. Ruggedized connectors have been developed as one way to accomplish lower cost and less complex FTTH networks.
Ruggedized optical fiber connectors are defined as optical fiber connectors that are designed for long-term performance in the outdoors (i.e., the outside-plant environment) without requiring enclosures, cabinets, or buildings for adequate protection from the outdoor elements (e.g., extreme hot and cold temperatures, solar UV radiation, rain, ice, snow, and wind).
The cost of FTTH network deployment can be reduced by initially installing the feeder and distribution cables of the network and subsequently making connections from the distribution cable to the home with pre-connectorized drop cables utilizing ruggedized connectors. This allows the cost of the last connection to be realized at the time the customer purchases the service (Internet access, cable television, HDTV, and video-on-demand).
Ruggedized connectors offer the benefit of making connections/disconnections at different times as needed to provide communications service without the capital equipment costs, operator skill level, and labor cost required for fusion splicing machines. Moreover, ruggedized connectors, unlike traditional connectors, do not require enclosures, cabinets, or terminals for protection from the outdoor elements, thereby further alleviating costs.
One example of such a ruggedized connector is the OPTITAP™ brand connector, commercially available from Corning Cable Systems. Ruggedized connectors can be installed directly onto single fiber cables making a connectorized cable assembly capable of reliable performance in the outside-plant environment without additional protection. In this regard, U.S. Pat. No. 6,579,014 disclosing a fiber optic receptacle and U.S. Pat. No. 6,648,520 disclosing a fiber optic plug are hereby incorporated by reference in their entirety.
For connectorized cables of higher fiber counts, however, there is a need to add a furcation tube between the end of the cable and the ruggedized connector. Furcation tubes allow multiple ruggedized connectors to extend from a multi-fiber drop cable. The currently available buffer tubes in loose tube cables and commercially available furcation tubes do not adequately protect the length of fiber extending from the cable to the ruggedized connector in the outdoor environment. Multi-fiber cables using currently available technology would require additional enclosures, cabinets, terminals, or buildings to adequately protect this furcated section of the assembly, thereby diminishing the benefit of ruggedized connectors.